Process for the improved production of (meth)acrylic acid esters of polyhydric alcohols (ii)

ABSTRACT

The invention relates to a process for the production of (meth)acrylic acid esters of polyhydric alcohols by reaction thereof with acrylic acid and/or methacrylic acid in the presence of acidic esterification catalysts with addition of polymerization inhibitors to the reaction mixture. The new process is characterized in that, where unsubstituted phenolic compounds are used as inhibitors, the esterification reaction is carried out with addition of active carbon to the reaction mixture. The preferred inhibitor is hydroquinone. In addition, the esterification reaction is preferably carried out in the absence of liquid solvents and/or azeotropic entraining agents.

This invention relates to a process for the production of polyfunctionalesters of acrylic acid and/or methacrylic acid--hereinafter referred toas (meth)acrylic acid esters or poly(meth)acrylic acid esters--withpolyhydric alcohols by reaction of the reactants in the presence ofacidic esterification catalysts with addition of polymerizationinhibitors to the reaction mixture.

(Meth)acrylic acid esters of polyhydric alcohols, particularly from thegroup of dihydric to tetrahydric aliphatic saturated alcohols and theiralkoxylation products, are being used to an increasing extent as highlyreactive constituents in radiation-curing systems. Polyfunctional(meth)acrylic acid esters of the type in question may be used, forexample, as paint constituents for hardening by electron beams or as aconstituent of UV-hardening printing inks or corresponding paints,surfacing, molding or encapsulating compounds or even in adhesives,particularly anaerobic adhesives. However, their production is notwithout problems. The end products are required in particular to becolorless with a low acid value, high stability in storage and hardlyany odor. (Meth)acrylic acid esters of the type in question generallycannot be purified by distillation on account of their high molecularweight and their high reactivity. Accordingly, the products are intendedto accumulate directly as colorless products of the esterificationreaction. The esterification reaction requires the presence of highlyeffective inhibitors which, in turn, should not initiate any unwantedsecondary reactions, for example in the form of discoloration.

Extensive literature is available on the production of suchpolyfunctional (meth)acrylic acid esters of polyhydric alcohols, cf. inparticular DE-OS 29 13 218 and the relevant literature cited therein.Thus, it is known from DE-AS 12 67 547 and from the Journal "Chem. andInd." 18 (1970), 597, that polyfunctional (meth)acrylic acid esters canbe produced by azeotropic esterification of (meth)acrylic acid withpolyhydric alcohols in the presence of azeotropic entraining agents andalso acidic catalysts and polymerization inhibitors, such as phenols,phenol derivatives, copper, copper compounds or phenothiazine. Organicor inorganic acids or acidic ion exchangers are used as the acidiccatalysts, p-toluene sulfonic acid and sulfuric acid being preferred.The esterification reaction takes place in particular at temperatures inthe range from 40° to 120° C. Suitable azeotropic entraining agents forremoving the water of reaction are aliphatic or cycloaliphatic oraromatic hydrocarbons or mixtures thereof having boiling ranges withinthe stated temperature limits.

It is proposed in DE-OS 29 13 218 cited above to carry out theazeotropic esterification in the presence of at least one organic esterof phosphorous acid in addition to a phenol-based inhibitor. However,the reaction again has to be carried out in the presence of at least onealiphatic, cycloaliphatic and/or aromatic hydrocarbon boiling at 40° to120° C. The water of reaction formed is said to be azeotropicallyremoved from the reactor by this entraining agent. According to theExamples of this publication, the reaction time is put at 10 to 18hours.

The problem addressed by the invention is to establish reactionconditions for the esterification reaction in question which, on the onehand, enable the reaction time to be considerably shortened but which,on the other hand, do not adversely affect the quality of theesterification products formed, particularly their high color quality.In addition, the invention seeks to eliminate the need for comparativelycomplex inhibitor systems of the type described in DE-OS 29 13 218 citedabove. Another problem addressed by the invention is to enable theapplication inhibitor required in practice for the highly reactivesystems in question to be used simultaneously as a reaction inhibitor inthe synthesis of the polyfunctional (meth)acrylic acid esters.

The teaching according to the invention is concerned in particular withthe problems which arise in reactions of the described type whenunsubstituted phenolic compounds, particularly hydroquinone, are used assole inhibitor or at least as the principal inhibitor component. The useof hydroquinone as a polymerization inhibitor in practice may bedesirable from the application standpoint. When used as a reactioninhibitor, hydroquinone causes problems, particularly in regard to thecolor of the reaction product form. The mere treatment of such highlydiscolored reaction products with, for example, active carbon as adecolorizing agent does not have the required lightening effect.Nevertheless, it has surprisingly been found that effective relief inthis regard can be provided by following the teaching according to theinvention as described hereinafter.

In one preferred embodiment, the technical solution provided by theinvention is based on the observation that that esterification productsof comparatively high purity can be directly obtained as end products ofthe process, even in the absence of diluents or azeotropic entrainingagents, and that it is even possible under the solventless reactionconditions to apply comparatively relatively drastic esterificationconditions which enable the reaction time to be considerably shortened.To this end, it is necessary in particular to carry out the processunder the conditions described hereinafter.

Accordingly, the present invention relates to a process for theproduction of (meth)acrylic acid esters of polyhydric alcohols byreaction thereof with acrylic acid and/or methacrylic acid in thepresence of acidic esterification catalysts with addition ofpolymerization inhibitors to the reaction mixture. The new process ischaracterized in that, where phenolic compounds unsubstituted in theα-position are used as inhibitors, the esterification reaction iscarried out with addition of active carbon to the reaction mixture. Thepreferred phenol-based inhibitor unsubstituted in the α-position ishydroquinone.

It has surprisingly been found that the use of active carbon during theactual condensation reaction--and not in a subsequent reactionstep--effectively prevents the formation of undesirably heavilydiscolored reaction products. The amount of active carbon used may beselected within wide limits and is preferably several times the quantityof the phenolic compound, i.e. above all hydroquinone, used asinhibitor. Although the color of the reaction product can besatisfactorily lightened with 2 to 10 times the quantity of activecarbon, the active carbon is nevertheless generally used in even largerquantities, for example of 10 to 100 times and preferably about 20 to 60times the quantity of hydroquinone. It had not been expected that theuse of such comparatively large quantities of active carbon would leavethe inhibitor effect against unwanted premature polymerizationsubstantially unimpaired. The known adsorptivity of the large surface ofthe active carbon evidently acts mainly against colored impuritiesformed during the reaction, removing them substantially selectively fromthe reaction mixture without, at the same time, impairing the effect ofthe phenolic compounds used basically in only small quantities aspolymerization inhibitor.

In another important and preferred embodiment of the invention, thereaction mixtures used are liquid at room temperature and are at leastsubstantially free from solvents and/or azeotropic entraining agents. Inthis embodiment of the invention, there is no need at all for suchauxiliaries which . . . the course of the reaction and, hence, basicallyreduce unwanted discoloration. Where this procedure is adopted, thewater of condensation formed is best removed from the gas phase of thereaction zone.

In another preferred embodiment of the invention, the reaction zone ispurged with a gas stream and the gas stream in question is used inparticular to remove the water of condensation formed during theesterification reaction from the reactor. It is preferred to use a gasstream which contains a limited amount of free oxygen. Depending on theparticular process conditions selected, air or an oxygen-depleted gasmixture, for example a nitrogen/air mixture, may be used as the gasstream. In general, however, a certain content of free oxygen will bedesirable in this gas phase delivered to the reaction mixture. Theselimited quantities of oxygen activate the inhibitor in known mannerduring the course of the reaction.

The oxygen content of the gas mixture is generally at least of the orderof 1% by volume and preferably in the range from about 2 to 20% byvolume. In the interests of reaction safety, the free oxygen contentsare preferably in the lower half of this range, i.e. up to about 10% byvolume and preferably up to about 7% by volume. In one preferredembodiment of the invention, the gas stream is fed into the liquidreaction mixture and can bubble through it, for example in finelydivided form. It is best to use limited quantities of this gas stream sothat there is no undesirably high discharge of reaction components,particularly the comparatively low-volatility acids.

On completion of the esterification reaction, the active carbon isremoved from the reaction mixture. This may be done in particular byfiltration. It is possible in this way to produce high-purity and, inparticular, substantially colorless radiation-curable polyfunctional(meth)acrylic acid esters, even under the comparatively drastic reactionconditions selected for the process according to the invention. Inaddition, the polyfunctional (meth)acrylic acid esters readily obtainedare distinguished by high stability in storage.

The polymerization inhibitor or, optionally, the inhibitor mixture istypically added to the reaction mixture in quantities of from 200 to10,000 ppm and preferably in quantities of from about 300 to 2,000 ppm,based in each case on the weight of the reaction mixture of(meth)acrylic acid and polyhydric alcohols.

Suitable polyalcohols for esterification are, for example, ethyleneglycol, propylene glycol, butane-1,4-diol, hexane-1,6-diol, neopentylglycol, diethylene glycol, triethylene glycol, dimethylol propane,glycerol, trimethylol propane, trimethylol hexane, trimethylol ethane,hexane-1,3,5-triol and pentaerythritol. According to the invention,however, particularly suitable polyhydric alcohols are also thealkoxylation products of the above-mentioned polyhydric alcohols,particular significance being attributed in this regard to theethoxylation products and/or propoxylation products. Chain-extendedpolyhydric alcohols of this type may contain considerable quantities ofpolyalkoxide groups, for example 1 to 50 mol and preferably about 1 to20 mol ethylene oxide per g-equivalent hydroxyl groups.

Suitable esterification catalysts for the process according to theinvention are commercially available organic or inorganic acids or evenacidic ion exchangers, particular significance being attributed to thecorresponding compounds frequently used in practice, namely p-toluenesulfonic acid and sulfuric acid. The esterification catalyst is used inquantities of, for example, from 0.1 to 5% by weight, based on theesterification mixture.

The reactants are preferably reacted at sump temperatures of at leastabout 90° C. and, preferably, of at least about 100° C., the temperaturerange up to about 150° C. being particularly suitable. The reaction maybe carried out under normal pressure, although it is best carried outunder reduced pressure. Where the reaction is carried out under reducedpressure, it is possible in one particular embodiment to reduce thepressure towards lower pressures either in steps or continuously.

Through the possibility of working under comparatively drasticesterification conditions and, at the same time, reduced pressure, thereaction time is considerably shortened by comparison with hithertoknown processes. Thus, yields of at least 90% of the theoretical and,preferably, of at least around 94% of the theoretical may be obtained inthe process according to the invention for a reaction time of no morethan about 10 hours and, preferably, of no more than about 8 hours attemperatures in the range from about 100° to 140° C. Nevertheless, thereaction products are obtained in the form of a stabilized mass which islight in color or may be effectively purified by a simpleaftertreatment.

The crude reaction product containing the acidic esterification catalystis subsequently subjected to neutralization. This neutralization stepmay be carried out under known wet conditions, for example by the use ofaqueous solutions containing soda and, optionally, sodium chloride. Inone preferred embodiment, however, the crude reaction product containingthe acidic catalyst is subjected to dry neutralization. Suitable dryneutralizing agents are the oxides and/or hydroxides of the alkalimetals, the alkaline earth metals and/or aluminium. Correspondingcompounds of magnesium or calcium are particularly suitable for the dryneutralization.

(Meth)acrylic acid and the alcohols may be used in equivalent quantitiesfor the esterification reaction. However, where more than dihydricalcohols are used, it is readily possible only partly to esterify thehydroxyl groups. For full esterification, it may be best to use the acidcomponent in a slight excess over the stoichiometric quantity requiredfor esterification of the hydroxyl groups. This slight excess may amountto at least about 10 mol-%. If desired, an inhibitor may be additionallyincorporated in the reaction product on completion of the reaction.

Should slight discoloration of the reaction product occur after allduring production under the drastic esterification conditions accordingto the invention, it may readily be eliminated by an aftertreatment withdecolorizing agents. Aluminium oxide, for example, is a suitabledecolorizing agent.

EXAMPLES Example 1

928.8 g acrylic acid, 1560.4 g of a propoxylated neopentyl glycol (OHvalue 460 mg KOH/g substance), 87.1 g p-toluene sulfonic acid and 124.5g (5%, based on acrylic acid+polyol) active carbon were weighed into a 3liter reactor and inhibited with 2.5 g hydroquinone (1100 ppm, based onthe quantity of product). An air/nitrogen mixture (5% by volume O₂ ; 20l/h) was passed through the reaction mixture during the esterificationreaction and water was removed. For a constant bath temperature of 143°C. and a maximum sump temperature of 135° C., the esterification timewas 5 hours. The mixture was cooled and the active carbon was filteredoff in a pressure nutsche.

Crude product:

Acid value: 34 mg KOH/g

OH value: 10 mg KOH/g

Yield: 96.7%

Gardner color standard number:<1

Viscosity: 92 mPas

The crude product was washed with 4 liters aqueous 16% by weight NaCl/4%by weight NaHCO₃ solution, reinhibited with 200 ppm hydroquinonemonomethyl ether, dried in vacuo for 3 hours at 80° C./4 mbar and thenfiltered in a pressure nutsche.

Product:

Acid value:<1 mg KOH/g

OH value:<15 mg KOH/g

Gardner color standard number: 3

Comparison Example 1

The procedure was as in Example 1, except that no active carbon wasadded.

Crude product:

Acid value: 39 mg KOH/g

OH value:<15 mg KOH/g

Yield: 97.1%

Gardner color standard number: 12

Viscosity: 90 mPa.s

The crude product was worked up as in Example 1.

Product:

Acid value:<1 mg KOH/g

OH value:<15 mg KOH/g

Gardner color standard number: 8-9.

Example 2

The procedure was as in Example 1, except that the active carbon wasreduced from 124.5 g to 24.9 g (1%, based on acrylic acid+polyol).

Crude product:

Acid value: 30 mg KOH/g

OH value:<15 mg KOH/g

Yield: 97.3%

Gardner color standard number: 6-7

The crude product was worked up as in Example 1.

Product:

Acid value:<1 mg KOH/g

OH value:<15 mg KOH/g

Gardner color standard number: 5-6.

                  TABLE 1                                                         ______________________________________                                        Dependence of the Gardner color standard number on the                        quantity of active carbon at different hydroquinone con-                      centrations in the esterification of acrylic acid with                        propoxylated neopentyl glycol                                                                Gardner color standard number                                  Quantity of A carbon                                                                         of the crude product with                                      x-fold excess, based                                                                         hydroquinone                                                   on hydroquinone                                                                              500 ppm      1000 ppm                                          ______________________________________                                        100            <1           --                                                80             <1           --                                                60             <1           --                                                50             --           <1                                                40             1-2          <1                                                30             --           1-2                                               20              5            5                                                10             --           6-7                                                0              12           12                                               ______________________________________                                    

Example 3

1198.8 g acrylic acid, 1362.3 g of an ethoxylated trimethylol propane(OH value 680 mg KOH/g substance), 89.6 g p-toluene sulfonic acid, 128.1g (5% by weight, based on acrylic acid+polyol) active carbon and 2.56 ghydroquinone (1100 ppm, based on the quantity of product) were weighedinto a 3 liter reactor.

An air/nitrogen mixture (5% by volume O₂ ; 20 l/h) was passed throughthe reaction mixture during the esterification reaction and water wasremoved. For a constant bath temperature of 143° C. and a maximum sumptemperature of 135° C., the esterification time was 5 hours. The mixturewas cooled and the active carbon was filtered off in a pressure nutsche.

Crude product:

Acid value: 25.3 mg KOH/g

OH value: 12 mg KOH/g

Yield: 96.3%

Gardner color standard number: 3

Viscosity: 78.9 mPas

The crude product was washed with 4 liters aqueous 16% by weight NaCl/4%by weight NaHCO₃ solution, inhibited with 200 ppm hydroquinonemonomethyl ether, dried in vacuo for 3 hours at 80° C./40 mbar and thenfiltered in a pressure nutsche.

Product:

Acid value:<1 mg KOH/g

OH value:<15 mg KOH/g

Gardner color standard number: 4

Comparison Example 2

The procedure was as in Example 3, except that no active carbon wasadded.

Crude product:

Acid value: 27 mg KOH/g

OH value:<15 mg KOH/g

Yield: 96.4%

Gardner color standard number: 11-12

The crude product was worked up as in Example 3.

Product:

Acid value:<1 mg KOH/g

OH value:<15 g KOH/g

Gardner color standard number: 7.

Example 4

The procedure was as in Example 3, except that the active carbon wasreduced from 128.1 g to 25.6 g (1% by weight, based on acrylicacid+polyol).

Crude product:

Acid value: 28.3 mg KOH/g

OH value:<15 mg KOH/g

Yield: 95.4%

Gardner color standard number: 8

The crude product was worked up as in Example 3.

Product:

Acid value:<1 mg KOH/g

OH value:<15 mg KOH/g

Gardner color standard number: 6-7.

                  TABLE 2                                                         ______________________________________                                        Dependence of the Gardner color standard number on the                        quantity of active carbon at different hydroquinone con-                      centrations in the esterification of acrylic acid with                        ethoxylated trimethylol propane                                                              Gardner color standard number                                  Quantity of A carbon                                                                         of the crude product with                                      x-fold excess, based                                                                         hydroquinone                                                   on hydroquinone                                                                              500 ppm      1000 ppm                                          ______________________________________                                        100            2            --                                                80             2            --                                                60             3            --                                                50             --           3                                                 40             5            3                                                 30             --           3-4                                               20             8            6-7                                               10             --           8                                                  0             11-12        11-12                                             ______________________________________                                    

We claim:
 1. In a process for the production of (meth)acrylic acidesters of polyhydric alcohols by reaction of at least one polyhydricalcohol with acrylic acid and/or methacrylic acid in the presence of anacidic esterification catalyst and a polymerization inhibitor in thereaction mixture, the improvement comprisingA. the use of at least oneα-unsubstituted phenolic compound as the polymerization inhibitor, andB. the addition of active carbon to the reaction mixture.
 2. The processof claim 1 wherein the quantity by weight of active carbon added to thereaction mixture is at least several times that of the at least oneα-unsubstituted phenolic compound.
 3. The process of claim 2 whereinfrom about 10 to about 100 times as much active carbon is added.
 4. Theprocess of claim 2 wherein from about 20 to about 60 times as muchactive carbon is added.
 5. The process of claim 1 wherein the reactionmixture is liquid at room temperature and is at least substantially freefrom solvents and azeotropic entraining agents.
 6. The process of claim1 wherein the water of condensation formed in the reaction is removedfrom the gas phase of the reaction zone.
 7. The process of claim 1wherein the reaction zone is purged with a gas stream containing freeoxygen.
 8. The process of claim 7 wherein air or a nitrogen/air mixtureis used as the gas stream.
 9. The process of claim 1 wherein thereaction is carried out at a sump temperature in the range of from about90° to about 150° C.
 10. The process of claim 9 wherein said temperatureis from about 100° to about 140° C.
 11. The process of claim 1 whereinthe reaction is carried out at a subatmospheric pressure.
 12. Theprocess of claim 1 wherein the α-unsubstituted phenolic compound ishydroquinone.
 13. The process of claim 1 wherein the at least oneα-unsubstituted phenolic compound is present in from about 200 to about10,000 ppm, based on the weight of the reaction mixture.
 14. The processof claim 13 wherein from about 300 to about 2,000 ppm of phenoliccompound is present.
 15. The process of claim 1 wherein the reaction iscarried out to a yield of product of at least 90% based on theoretical,but wherein the reaction time is not greater than about 10 hours. 16.The process of claim 1 wherein the reaction product is subjected to dryneutralization.
 17. The process of claim 16 wherein the dryneutralization is carried out with at least one oxide or hydroxide of analkali metal, an alkaline earth metal, or aluminum.
 18. The process ofclaim 1 wherein the reaction product is treated with a decolorizingagent.
 19. The process of claim 1 wherein the quantity by weight ofactive carbon added to the reaction mixture is at least several timesthat of the at least one α-unsubstituted phenolic compound; the reactionmixture is liquid at room temperature and is at least substantially freefrom solvents and azeotropic entraining agents; the water ofcondensation formed in the reaction is removed from the gas phase of thereaction zone; the reaction zone is purged with a gas stream containingfree oxygen; and the reaction is carried out at a sump temperature inthe range of from about 90° to about 150° C.
 20. The process of claim 19wherein the reaction is carried out at subatmospheric pressure.
 21. Theprocess of claim 19 wherein the reaction temperature is from about 100°to about 140° C.; the quantity of active carbon is from about 10 toabout 100 times that of the at least one α-unsubstituted phenoliccompound; the at least one α-unsubstituted phenolic compound is presentin from about 200 to about 10,000 ppm, based on the weight of thereaction mixture; and the reaction is carried out to a yield of productof at least 90% based on theoretical, but wherein the reaction time isnot greater than about 10 hours.
 22. The process of claim 21 whereinfrom about 300 to about 2,000 ppm of phenolic compound is present; airor a nitrogen/air mixture is used as the gas stream; and the reactionproduct is subjected to dry neutralization.